The conventional fixed-frequency pulse width modulation (PWM) controller has poor transient response. Recently, people devote to develop variable-frequency PWM controller system to overcome this issue. For the ultra-fast transient response characteristic, however, the output capacitor of a variable-frequency PWM controller system is smaller than that of a fixed-frequency PWM controller system, and another problem, noise sensitivity, is introduced eventually. The popular variable-frequency PWM controller senses the ripple of the output voltage of the switching system to be a ramp to compare with a reference so as for its system control. For example, as shown in FIG. 1, a switching system 10 comprises an error amplifier 102, a constant on-time circuit 104, two drivers 106 and 108, an output stage 110, and a voltage sense circuit 112, among which the output stage 110 includes a high-side switch SW1 connected between a supply voltage VDD and a phase node 114, a low-side switch SW2 connected between the phase node 114 and ground GND, an inductor L connected between the phase node 114 and the output Vout, and an output capacitor C having an equivalent series resistance (ESR) RESR connected between the output Vout and ground GND. The voltage sense circuit 112 senses the output voltage Vout to generate a feedback signal FB, the error amplifier 102 receives the feedback signal FB to compare with a reference VREF to generate a signal PM, and the constant on-time circuit 104 controlled by the signal PM generates an on-time signal Ton for the drivers 106 and 108 to drive the switches SW1 and SW2, such that an inductor current IL is generated to charge the capacitor C through the inductor L to provide the output voltage Vout to a load RL. As the feedback signal FB is lower than the reference VREF, it will trigger a constant on time to the driving signal UG for the high-side switch SW1 by the constant on-time circuit 104. Following the falling edge of the on time duty, a minimum off time is triggered to keep the system stable. Responsive to a load transient, the output voltage Vout will drop a value equal to the product of RESR and IL instantly, and this voltage drop will apply to the error amplifier 102, such that the driving signal UG turns on in the shortest response time. The topology has fast transient response, but induces noise sensitivity problem.
FIG. 2 shows a timing diagram of various signals in the circuit 10 of FIG. 1 during a load transient, in which waveform 202 represents the inductor current IL, waveform 204 represents the feedback signal FB, waveform 206 represents the reference VREF, waveform 208 represents the signal PM, waveform 210 represents the on-time signal Ton, waveform 212 represents the minimum off-time signal Toffmin, and waveform 214 represents the driving signal UG generated by the driver 106. At time t, a load transient is occurred, and thereby the inductor current IL begins to increase. As a result, the output voltage Vout drops rapidly, and eventually, the feedback signal FB also drops rapidly. By the error amplifier 102, the duty of the signal PM is enlarged, and by which the constant on-time circuit 104 increases the frequency of the on-time signal Ton, the frequency of the driving signal UG is increased accordingly, such that the high-side switch SW1 is switched more frequently to recover the output voltage Vout to its original level as soon as possible. However, in reality, the feedback signal FB involves high-frequency noise, as shown in FIG. 3 by waveform 204 indicated by noisy portions 218 and 220 for example. Moreover, to achieve fast transient response, the variable-frequency PWM system 10 uses the output capacitor C much smaller than that in a fixed-frequency PWM system, and thus the system becomes more sensitive to noise. When the noise near the valley of the waveform 204 is so large to pull the feedback signal FB lower than the reference VREF, error operation may occurred. For example, as shown in the portion 220, the high-frequency noise near a valley of the waveform 204 is large enough for the feedback signal FB lower than the reference VREF incorrectly, resulting in error operation or malfunction of the system 10, as shown in the driving signal UG corresponding to the portion 220.
Therefore, it is desired an apparatus and method for noise sensitivity improvement and fast transient response for a switching system.